The present invention relates generally to inkjet printing mechanisms, and more particularly to an optical system for determining ambient humidity where an inkjet printing mechanism is operating, so printing routines may be adjusted to provide fast, high quality output in all environments.
Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are described and shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. To facilitate priming, some printers have priming caps that are connected to a pumping unit to draw a vacuum on the printhead. During operation, partial occlusions or clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a clearing or purging process known as xe2x80x9cspitting.xe2x80x9d The waste ink is collected at a spitting reservoir portion of the service station, known as a xe2x80x9cspittoon.xe2x80x9d After spitting, uncapping, or occasionally during printing, most service stations have a flexible wiper, or a more rigid spring-loaded wiper, that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solids content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper.
Various environmental factors affect inkjet printing routines, servicing routines, and other aspects of printer performance. Unfortunately in the past, there has been no way to economically provide an environmental factor input to a printer controller to allow the controller to modify these printing, servicing and other routines to provide optimum performance in light of the current environmental conditions. One environmental factor, temperature, may currently be monitored using temperature sensing resistors within the inkjet printheads; however, more important to printer performance than temperature is the environmental factor of humidity. Unfortunately, the currently available humidity sensors are far too expensive for the home and small business inkjet printing markets, with manufacturer""s material costs for capacitive sensors ranging several dollars per sensor not including the cost of their support electronics, while voltage output humidity sensors currently cost about ten dollars each. Moreover, the currently available capacitive humidity sensors are inaccurate, so their inaccuracy coupled with their high cost renders their use unjustifiable in the home and small business inkjet printing market.
If humidity could be both economically and accurately measured for communication to a printer controller, a variety of performance enhancements could be made based upon knowledge of the ambient humidity. For example, presently to provide optimum performance in varying environmental conditions, inkjet printing, servicing, and other routines are based on a xe2x80x9cworst case scenarioxe2x80x9d assumption of the environmental conditions, here meaning a high humidity environment for printing and a low humidity environment for printhead servicing, as well as for vapor transfer calculations which account for ink evaporation from the pens. The terms xe2x80x9cdryxe2x80x9d and xe2x80x9chumidxe2x80x9d are used herein to assist the reader in understanding which end of the scale refers to which condition. For instance, a xe2x80x9cdryxe2x80x9d condition normally is associated with a desert environment, whereas a xe2x80x9chumidxe2x80x9d condition is normally associated with a tropical environment, although it is apparent that during a cloud burst a desert may become a very humid environment for a short period of time.
In high humidity conditions, the media may already be moist and partially saturated before ever being loaded into a printer, and high humidity increases the drying time of aqueous-based inks. These high humidity conditions may lead to increased cockle of the media, a term referring to the swelling of the paper fibers when saturated with ink, causing a buckling which in extreme conditions may cause the media to buckle so high that the printhead crashes into the media, smearing the printed image and possibly damaging the printhead. Thus, a high humidity assumption increases the dry time delay for the media over that required in normal or low humidity conditions, which slows media throughput while a printer waits for one sheet to dry before depositing the next sheet on top of the previously printed sheet in the output tray. Furthermore, the low humidity assumptions for servicing increase the duration of servicing routines, which further slows media throughput.
Low humidity conditions contribute to hue shift problems, where various components of the ink evaporate over time, for instance by leaking at the printhead/cap sealing interface. In xe2x80x9coff axisxe2x80x9d printing systems, where the printheads carry only a small supply of ink across the printzone and are replenished with ink delivered from a stationary main ink reservoir through flexible tubing, some of the ink volatiles leach through the tubing walls to atmosphere. Any loss of one ink component changes the ink composition, resulting in changes in ink performance, often manifested as a hue shift in the resulting image. For instance, with fewer volatiles, the resulting ink dispensed by the printhead has a higher concentration of dyes or colorants, yielding a darker image than originally intended. To compensate for these ink composition changes, ambient humidity information may be used for vapor transfer rate calculations to allow for hue adjustment based on calculated dye load changes over time within the inkjet cartridges.
As another example of the impact of this high humidity assumption on printer performance, when performing duplex printing one typical duplexer unit typically holds a sheet after printing the first side for nearly seven seconds before reversing the sheet and beginning printing on the opposite surface. In low humidity conditions, such as in a desert setting, holding a sheet of paper for seven seconds as one would in a humid region unnecessarily delays duplex printing. These same delays are incurred to avoid cockle problems when printing single sided sheets. For pen servicing, it would be desirable to know the ambient humidity so the type of servicing routine performed on the printheads following uncapping and before a print job may be optimized. Additionally, by knowing a humidity history of the printer, vapor transfer rate calculations may be made to determine the amount of ink lost due to evaporation, which then may be used in conjunction with drop counting or other measures to predict when an inkjet cartridge is nearing an empty condition, allowing an operator to be warned before the cartridge runs dry.
Clearly, a variety of different printing, servicing and other performance operations may be adjusted and optimized if only the ambient humidity were input to the printing mechanism. Thus, one goal herein is to provide an ambient humidity input to an inkjet printing mechanism, which may use this input to optimize printer performance to provide fast high quality hard copy outputs.